This invention relates to electrochemical cells and in particular although, not exclusively to electrochemical cells for use in gas sensors and fuel cells.
An electrochemical gas sensor for sensing an oxidisible or reducible gas (e.g. carbon monoxide) in the atmosphere usually contains a sensing or working electrode, a counter electrode and an inlet (usually a diffusion barrier) to allow the atmosphere to permeate to the sensing electrode. Both electrodes are in contact with an electrolyte in order firstly to produce an electrochemical reaction at the sensing electrode with the gas to be sensed, and secondly to produce an electrochemical reaction at the counter electrode with oxygen in the atmosphere, electrolyte or other gas source. Current is carried through the electrolyte by ions produced in the reaction and by electrons through an external circuit, the current in the circuit indicating the gas concentration. A reference electrode may be employed in combination with a potentiostat circuit to maintain the potential between the sensing electrode and the cell electrolyte in order to increase stability of operation.
In terms of physical construction, the sensor normally comprises an external housing which acts as a reservoir for the electrolyte, a wick or matrix to hold the electrolyte in contact with the electrodes, and external electrical terminals making electrical connection with the electrodes. The majority of present sensor cells use a stacked electrode arrangement, as for example in U.S. Pat. No. 4,406,770.
There has been recent proposals for design simplification, see for example U.S. Pat. No. 5,183,550 which discloses a gas sensor in which the sensing, counter and reference electrodes are mounted in a common plane on a common ceramic substrate, with contact leads extending from the electrodes to the other surface of the substrate for electrical connection.
Our copending application WO 96/14576 (our ref. PQ12622) discloses and claims a gas sensor comprising a substrate, electrodes formed as porous planar elements on the substrate, and the substrate being porous to permit permeation of gas to the electrodes from the environment, a housing containing a reservoir of electrolyte and external terminals mounted to the housing, wherein the substrate is bonded to the housing by the application of pressure and heat, so that in a single assembly operation, the housing is sealed and electrical connections are made to the electrodes while blocking the porosity of the electrodes to prevent electrolyte permeating through the electrode to the area of the electrical connection.
While the above construction represents a considerable advance in terms of cost reduction, nevertheless inexorable demands for cost reduction without sacrificing quality, forces further design simplifications.
This invention is based on a concept of providing two flexible plastics sheets, one having electrode areas printed thereon, which are bonded together around their edges to form a reservoir for electrolyte, rather in the form of a tea bag.
Accordingly, the present invention provides in a first aspect, an electrochemical cell comprising at least first and second sheet members, at least one of the sheet members including a gas permeable section on which is disposed one or more planar electrodes, peripheral regions of the first and second sheet members being sealed together to form a reservoir containing electrolyte, and including electrical connection means extending from each of said electrodes across the sealing of the sheet members for external electrical connection.
In a further aspect, the invention provides an electrochemical cell assembly formed of a plurality of sections, each section having first and second, sheet members, each sheet member including a gas permeable region on which is disposed a planar electrode, peripheral regions of the first and second sheet members being sealed together to form a reservoir containing electrolyte, and including electrical connection means extending from said electrodes across the sealing of the sheet members for external electrical connection, said assembly including manifold means for directing first and second gases to each section so as to contact respective first and second sheet members.
In the latter embodiment of the invention, by forming the electrodes on different sheet members, it is possible to arrange a first gas to flow over the first sheet member, and a second gas over the second sheet member. For example with a suitable manifold structure, the assembly may constitute a fuel cell.
The construction of the present invention, either as a gas sensor or fuel cell, is extremely simple and permits substantial cost savings.
Various specific forms of construction are possible. Both sheet members may be flexible, the flexibility permitting the insertion of electrolyte between them. Both sheet members may be part of a single sheet, which is folded over so that the sheet members are face to face. Alternatively one or both sheet members may be formed from a sheet which is performed to have a three dimensional shape, for example one sheet may have a well formed therein to define a reservoir space. Alternatively in one preferred construction, one sheet may be formed as a planar sheet of porous PTFE carrying the required configuration of electrodes, and a flexible plastics closure sheet may be welded to the edges of the PTFE sheet to define the reservoir.
A third sheet member may also be employed, for example an intermediate layer of lower melting point for sealing the two outer sheets. The third sheet may be of highly porous material to hold the electrolyte. In a further form, the third sheet member may define a second reservoir space for a second electrochemical cell.
The invention thus permits very thin assemblies (of the order of 1 to 2 mm) to be produced in a variety of shapes. This permits applications for a gas sensor where space is extremely limited, e.g. on or in a person""s clothing. The extreme cheapness of production provides the possibility of xe2x80x9cdisposablexe2x80x9d sensors which may be used only once or a small number of times and then disposed of.
Thus production and assembly may be simplified by printing the electrodes on a sheet of flexible substrate material which then is folded over one or more times, or placed against another sheet of material and then sealed around the edges to form a xe2x80x9ctea bagxe2x80x9d type structure. The wick and electrolyte are contained within the bag and the sealing may be by heat, adhesive or mechanical force. Such devices may be made in irregular shapes, and are suitable for high levels of automation for cost reduction.
Where sensors of a completely flexible construction are produced, they will in practice normally be attached to a rigid support or mounted in a rigid housing to prevent spurious noise due to bending. Due to the compact nature of the thin sensor, it may be disposed with control electronics on to a single substrate. This substrate may also have the diffusion-limiting gas access built in.
The electrochemical cells in accordance with the invention lend themselves to automatic fabrication and assembly on a production line. Thus sheet material stored in one or more rolls can be unwound and superimposed, and a pattern of cells then pressed, cut and sealed in simultaneous operations from the sheets. For a fuel cell, the cells may be formed as flat arrays, e.g. 4xc3x974 on the sheets.
While the electrolyte in the cells is usually in the form of a liquid, it may be in the form of a gel or solid polymer, pasted or otherwise affixed to the electrodes.
For an electrode printed on a gas permeable membrane three functions have to be achieved to assemble it into an electrochemical cell: 1) to mechanically attach the electrode, 2) seal in the electrolyte and 3) to provide a conductive path from the electrode to the outside of the cell.
A preferred construction method, similar to that as described in WO 96/14576 uses the technique of heat sealing a porous PTFE sheet member (the electrode) to the cell body component As the printed electrode runs through this seal all three needs are met with the addition of no extra components and in addition the ability to automate is provided.
The sheet members may also be glued together; this method can achieve all three requirements for cell assembly. In practice it makes use of the porosity of a PTFE membrane to achieve adhesion. This method of construction is important to allow the assembly of fragile or complex electrodes. Fragile electrodes may result from cost reduction, for example. Complex electrodes include irregular shapes, multiple prints or sealing in more than one plane e.g. around the outside of a moulding. This assembly method may be important for small disposable sensors (limited life). Advantages: very little disruption of the electrode ink structure, the adhesive can enter the porosity of the electrode ink and substrate increasing strength, and the method is very adaptable. With this method, different methods of electrical connection to the electrodes may be employed, for example electrode lead wires extending from the electrodes to external electrical connections.
The ability to produce multiple cell assemblies at low cost is ideal for fuel cell fabrication. The production of cells in a strip form may be adapted to give an array (say 3xc3x974), this array forming a single layer in a stacked assembly. One electrode of each cell is on the top of the layer and the other underneath, to allow the air and fuel gases to be supplied via simple manifold systems to the relevant electrode.
A preferred stacked assembly includes rigid spacer devices interleaved with the layers of cells and providing a manifold structure on each side of each layer to permit inflow of an appropriate gas. First manifolds on one side of the layer are disposed to allow inflow of a fist gas from one direction and second manifolds are disposed on the other side of the layer are arranged at right angles to the first manifolds to allow inflow of a second gas from a direction at right angles to the first By adjusting the size and shape of the manifolds the volume or flow rate of the two gases could be set as desired.
In each layer the cells may be connected in series to dictate the voltage generated by the cell, and the layers may be connected together in parallel in order to define the maximum current of the assembly.
A second stack assembly is to have all the layers fabricated in one long strip which is folded up in a xe2x80x9czigzagxe2x80x9d pattern. A rigid manifold/spacer arrangement is employed to permit air flow through the stack at right angles to the fold pattern.
In the case of a fuel cell, it is possible that the sheet members may not be completely sealed around their peripheries, but that a small unsealed region may be provided in order to provide a vent for water or other liquid generated during operation within the cell.